Many types of electronic devices include data cells having a single transistor. Typically, the transistor controls whether a stimulus (e.g., a current or voltage) is applied to, or by, a data element (e.g., a memory element, an imaging element, or other device configured to output data, such as various kinds of sensors). Often a large number of data elements are disposed in an array, and the transistor allows individual data elements in the array to be selected. For example, certain types of dynamic random access memory (DRAM) cells include both a capacitor, which functions as a data element, and a single transistor, which functions as an access device, connected to the capacitor. The capacitor usually stores data by storing a charge that is representative of data (e.g., a 0 or a 1 in a single-bit device, or a 00, 01, 10, or 11 in a two-bit device), and the transistor typically controls access to the capacitor by controlling the flow of current to and from the capacitor, allowing current to flow during reading and writing and preventing current from flowing when retaining data.
Often the data elements are arranged in an array, e.g., generally in rows and columns. Data cells within the array are accessed, e.g., written to or read from, through circuitry near the periphery of the array. For instance, sense amplifiers or other sensing circuitry are often positioned adjacent to arrays of data cells for reading data. Similarly, address decoders, e.g., row and column address decoders, are often disposed adjacent to the array for addressing particular data cells or groups of data cells.
As the footprints of such devices become smaller, the components of the device may become smaller and/or denser for a given storage capacity. Additionally, some structures may be more vertical (i.e., less planar with respect to the substrate) to reduce footprint size. In such devices, construction of the data elements and the support structures (e.g., digitlines, wordlines, etc.) may present challenges and may limit scaling such devices to smaller footprints and higher densities.
Further, in conventional device formation, polysilicon layers are used as gate electrodes. However, the polysilicon layers must be doped in order to achieve a desired work function. Unfortunately, polysilicon gate electrodes suffer from a depletion effect. The depletion effect occurs when the portion of the gate electrode nearest an underlying oxide layer is depleted of dopants, causing the gate electrode layer to behave like an insulating layer, leading to device degradation and eventual malfunction. Silicon oxide is commonly used as an insulating layer. A thinner silicon oxide is required to increase gate capacitance, causing high drive current. However thin silicon oxide causes high gate leakage current, leading to excessive power consumption in mobile devices. For high performance CMOS applications where high dielectric constant (high-k) layers are applied as a replacement for silicon oxide, the polysilicon gate interacts with the high-k film because of the Fermi pinning effect, resulting in a high threshold voltage and poor transistor drive performance. Metal layers are needed as gate electrodes if high-k dielectric layers are used for CMOS transistors. The work function close to Si band edges is effective to achieve low threshold voltage for CMOS transistors. However the work function shifts to the band middle from the band edge if a high temperature treatment is performed after the formation of the gate electrode.
Therefore, there is a need in the art for a method and apparatus for fabricating a metal/high-k gate stack with a low temperature process for a buried recessed access device.